The present invention relates to processes for heat treating ferrous material and, more particularly, to methods for annealing, normalizing, spherodizing, etc. rod and wire under nitrogen based atmospheres.
In annealing ferrous material, such as for purposes of stress relief, spherodizing, annealing, etc., it is important to avoid changes in the surface metallurgy of the material. Thus, surface carburization and decarburization are to be avoided during annealing and other similar heat treating processes. It has been found that ferrous material such as rod and wire exhibit a tendency to decarburize during annealing even at relatively low temperatures as a consequence of leakage into the furnace of decarburizing agents such as CO.sub.2 from the ambient atmosphere. It has been common practice to utilize protective atmospheres such as endothermic, exothermic, or others in heat treating furnaces. However, as these atmospheres are derived from hydrocarbon sources such as natural gas, the cost of producing these atmospheres has increased significantly in recent years. Furthermore., expensive generator devices are necessary to produce these atmospheres, and these devices require considerable maintenance but yet are relatively inflexible in that they are not effective to produce atmospheres of variable compositions over a wide range of flow rates. Typically, the foregoing conventional generated atmospheres are enriched with natural gas or pure methane so that an adequate level of hydrocarbon is available to react with oxidizing and decarburizing agents leaking into the furnace to thereby avoid decarburizing or oxidation of the ferrous material being annealed or otherwise treated.
In order to avoid carburization or decarburization of ferrous material at a particular temperature in a furnace, it is necessary to maintain an equilibrium condition between carbon dioxide and carbon monoxide, i.e. maintain a predetermined ratio at a given temperature. A discussion of these ratios appears in WIRE TECHNOLOGY, November-December, 1979, pages 51-57 which indicates that by adding a hydrocarbon to a nitrogen based atmosphere, the level of CO increases by virtue of the reactions: EQU CH.sub.4 +CO.sub.2 .revreaction.2CO+2H.sub.2 ( 1) EQU 2CH.sub.4 +3O.sub.2 .revreaction.2CO+4H.sub.2 O (2) EQU CH.sub.4 +H.sub.2 O.revreaction.CO+3H.sub.2 ( 3)
Such an increase in CO tends to reduce the ratio of CO.sub.2 /CO at a constant temperature to thereby reduce the decarburizing tendency of the furnace atmosphere. Thus, methane addition is effective to control or limit the tendency of a furnace atmosphere to decarburize, but this reaction yields only two molecules of CO for each molecule of methane supplied.
It has been proposed, for example in U.S. Pat. No. 4,415,379, which assigned to the assignee of the present invention, to introduce both propane and methane into a nitrogen based furnace atmosphere in an annealing process. It has been found that this combination of reactants results in the breakdown of propane into a relatively active form of methane which tends to preclude decarburization at lower temperatures. However, the remaining propane will react with CO.sub.2, O.sub.2 and H.sub.2 O in accordance with the following formulas: EQU C.sub.3 H.sub.8 +CO.sub.2 .revreaction.2CO+2CH.sub.4 ( 4) EQU C.sub.3 H.sub.8 +3.5O.sub.2 .revreaction.3CO+4H.sub.2 O (5) EQU C.sub.3 H.sub.8 +3H.sub.2 O.revreaction.3CO+7H.sub.2 ( 6)
Thus, propane is also effective by such a reaction to produce two or three molecules of carbon monoxide for each molecule of propane supplied to the furnace.
In European Patent Application, publication No. 027649, published Apr. 29, 1981, it is proposed to utilize methanol as an additive in a nitrogen based annealing process. Although this reference suggests that methanol is effective to avoid carburization and decarburization, it is noted that methanol will react in such atmospheres as follows: EQU CH.sub.3 -OH.revreaction.CO+2H.sub.2 ( 7) EQU H.sub.2 +CO.sub.2 .revreaction.CO+H.sub.2 O (8)
Thus, although methanol may be effective as an addition to nitrogen based furnace atmospheres during annealing processes, only two molecules of CO are produced from each molecule of methanol supplied to the furnace.
In addition to the foregoing, it has been proposed to utilize materials such as ethane, ethylene, and acetylene in heat treating processes as is described in U.S. Pat. No. 4,108,693. Also, U.S. Pat. No. 4,016,011 describes processes for heat treating ferrous material wherein a liquid organic compound such as methylacetate or isopropyl alcohol is utilized with a nitrogen based atmosphere. It is noted, however, that the reactions resulting as a consequence of utilizing such agents in heat treating processes do not yield levels of carbon monoxide significantly greater than those described above in connection with more conventional additives such as methane, propane, etc.
It is also known to utilize hydrocarbons in the form of propylene (C.sub.3 H.sub.6) in the course of annealing high carbon steel in nitrogen based atmospheres as is described in HEAT TREATING, March, 1982, pages 32-34. The reactions of propylene with CO.sub.2, O.sub.2 and H.sub.2 O occurs as follows: EQU 2C.sub.3 H.sub.6 +3CO.sub.2 .revreaction.6CO+3CH.sub.4 ( 9) EQU C.sub.3 H.sub.6 +3O.sub.2 .revreaction.3CO+3H.sub.2 O (10) EQU C.sub.3 H.sub.6 +3H.sub.2 O.revreaction.3CO+6H.sub.2 ( 11)
In U.S. Pat. No. 4,154,629, the use of an organic liquid as an additive to nitrogen based atmospheres for heat treating ferrous material involves the production of three molecules of CO for each molecule of the organic liquid supplied to the heat treating furnace as is the case from the above reaction of propylene with CO.sub.2, O.sub.2 and H.sub.2 O.
Thus, there is a clear need for processes for annealing, normalizing and spherodizing ferrous material wherein decarburization and oxidation of such material is essentially avoided.